The ultimate breakthrough would be an opening of China’s domestic stock market to foreign investors — into what’s known as “A shares.” Foreign investors today are only allowed into certain types of Chinese stocks. A complete opening would have a dramatic impact upon outsiders, not to mention stock prices in China.

Believe it when you see it, I guess. But today, we heard an interesting MSCI Inc. (MSCI) estimate of the impact an opening would have on the MSCI Emerging Markets Index. This benchmark, the best known of its type, is the basis for widely owned funds including the iShares MSCI Emerging Markets Index Fund (EEM).

It would be pretty dramatic. With a weighting today around 18%, the index’s tilt toward China would jump to 30% in the hypothetical event that China’s A-share market opened completely to foreigners, according to Diana Tidd, head of the MSCI Americas index business.

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